Today when every major nation of the world is doing their best to spy on each other and break each others secret codes, it is difficult to believe that there was once a US Secretary of State who declared "Gentlemen do not read each other's mail." Yet it was this statement by US Secretary of State Henry L. Stimson in 1929 that led to this book because the result was that the chief codebreaker for the USA Herbert O. Yardley lost his job and was cast out on the streets. Having no way to make a living other than playing poker, Yardley decided to pick up some cash by writing this book, which proved to be a great success. If somebody in Yardley's position were to write a book like this today, he would have to flee the country and seek the protection of a foreign government as Edward Snowden has done. However, Yardley was able to write this book and get away with it, but just barely as his next book was seized by the US government and not allowed to be published. This book contains invaluable information on codes and code breaking, including how to make a code and how to break a code. We learn fantastic things. For example, a married woman might write a letter in code to her lover to prevent her husband from finding out about it. If the code is intercepted and broken, it might appear to be a simple and harmless love letter. However, that might conceal a deeper code hidden within it that reveals military secrets. Another aspect of code breaking is finding the foreign government's code book. This may be accomplished by the use of a Mata Hari, a woman who pretends to fall in love with the official of the foreign government, who sleeps with him and thus is able to get access to his code book. One would think it would be difficult to find a woman willing to do this kind of work. However, according to Yardley, there are many upper-crust women willing to do this. Almost all of them will do it. The risk is that after being assigned to the task of falling in love with the man, she really does fall in love with him and thus stops transmitting his coded messages to Yardley. This is the history of an era when the United Sates stood on the brink of another war with Mexico. We learn that Yardley's team intercepted and decoded more than five thousand messages from Germany to Mexico during a period when Germany was trying to convince Mexico to declare war against the United States, offering the support of Germany and offering the states of Arizona, New Mexico and Texas to Mexico if it was successful.